1. Field of the Invention
The present invention relates to a method for reducing the wear of at least one metal and/or ceramic surface that is in contact with an advanced ceramic surface which comprises contacting said surfaces with a lubricating oil containing an oxidant such as hydroperoxide.
2. Description of Related Art
Wear is the primary factor that limits the life of an engine. Valve train represents one of the highest wear components within an engine, particularly for the newer models that have smaller but higher speed engines which incorporate current designs such as overhead camshafts, 4-valves per cylinder and turbochargers. In view of the trend toward extended drive train warranties, there is an increased demand for lubricants with improved antiwear performance. In a parallel effort, automotive engine and component manufacturers are enhancing valve train durability with emerging technologies such as roller followers and the use of advanced ceramic materials.
The use of advanced ceramics in high wear locations such as the valve train is attractive since they provide a potential for enhancing not only engine durability, but also performance. Among the ceramics used in engine applications are aluminum oxide (Al.sub.2 O.sub.3), partially stabilized zirconium oxide (PSZ), silicon carbide (SiC), silicon nitride (Si.sub.3 N.sub.4) and their hybrid combinations, all of which potentially offer a high degree of mechanical integrity, chemical inertness, thermal stability and hardness that would impart good wear resistance. Because of their low densities, ceramics can decrease inertial loads, which permits the use of valve springs with lower rates without causing valve float. This ultimately reduces the contact stresses at wear points.
Oxidants such as hydroperoxides are not normally added to engine lubricating oils because they facilitate degradation of the oil and promote engine wear (see Habeeb et al., "The Role of Hydroperoxides in Engine Wear and the Effect of Zinc Dialkyldithiophosphate," ASLE Transactions, Vol. 30, No. 4, pp. 419-26 (1987) and Habeeb et al., "The Role of Hydroperoxide in Engine Wear and the Effect of ZnDDP/Dispersant/Detergent Interactions," SAE Paper No. 872157 presented at the International Fuels and Lubricants Meeting and Exposition in Toronto, Ontario, Canada (Nov. 2-5, 1987), the disclosures of which are incorporated herein by reference). Therefore, it is surprising that reduced wear of a metal and/or ceramic surface in contact with an advanced ceramic surface can be obtained by contacting said surfaces with a lubricating oil containing an oxidant.